Selective acetylene hydrogenation

ABSTRACT

A catalyst composition comprising palladium, silver and a support material (preferably alumina) is contacted at a relatively low temperature (of up to about 60° C.) with a liquid composition comprising an effective reducing agent (preferably an alkali metal borohydride, hydrazinc, formaldehyde, formic acid, ascorbic acid, dextrose, aluminum powder). Preferably, at least one alkali metal compound (more preferably KOH, RbOH, CsOH, KF) is also present in the liquid composition. An improved process for selectively hydrogenating acetylene (to ethylene) employs this wet-reduced catalyst composition.

This application is a division of U.S. patent application Ser. No.08/269,723, filed Jul. 1, 1994.

BACKGROUND OF THE INVENTION

In one aspect, this invention relates to a method of making supportedpalladium/silver compositions exhibiting improved acetylenehydrogenation catalyst performance. In another aspect, this inventionrelates to a process for selectively hydrogenating acetylene to ethyleneemploying supported palladium/silver catalysts having been prepared bythe preparation method of this invention.

The selective hydrogenation of acetylene being present as an impurity inmonoolefin-containing streams (e.g., ethylene streams from thermalethane crackers) is commercially carried out with an alumina-supportedpalladium/silver catalyst, substantially in accordance with thedisclosure in U.S. Pat. No. 4,404,124 and its division, U.S. Pat. No.4,484,015. The operating temperature for this process is selected suchthat essentially all acetylene is hydrogenated to ethylene (and thusremoved from the feed stream) while only an insignificant amount ofethylene is hydrogenated to ethane (to minimize ethylene losses and toavoid a "runaway" reaction which is difficult to control, as has beenpointed out in the above-identified patents). The selective acetylenehydrogenation process can be most effectively controlled when there is alarge difference between the temperature at which essentially allacetylene is hydrogenated and a higher temperature at which excessiveethylene-to-ethane conversion occurs. Even though the Pd/Ag/Al₂ O₃catalyst described in the above-identified patents is an excellentcatalyst, the present invention represents an improvement in thepreparation of this catalyst and related catalysts and their use for theselective acetylene hydrogenation.

SUMMARY OF THE INVENTION

It is an object of this invention to prepare improvedpalladium/silver-containing catalyst compositions which are useful forcatalyzing the selective hydrogenation of acetylene to ethylene. It isanother object of this invention to carry out the selectivehydrogenation of acetylene to ethylene in the presence of improvedpalladium/silver catalysts. It is a further object of this invention toemploy these improved catalysts for hydrogenating acetylene which ispresent in small amounts in ethylene-containing streams. Other objectsand advantages will be apparent from the detailed description andappended claims.

In accordance with this invention, a preparation method comprisescontacting a solid composition (also referred to as "base catalystcomposition" or "starting catalyst composition") comprising palladium,silver and an inorganic support material with a liquid compositioncomprising at least one effective reducing agent at a temperature of upto about 60° C., at contacting conditions which are effective inenhancing the selectivity of said solid catalyst composition whenemployed as a catalyst for hydrogenating acetylene to ethylene. In apreferred embodiment, said contacting is carried out with a reducingagent in the presence of at least one alkali metal compound, morepreferably an alkali metal hydroxide and/or an alkali metal fluoride.

In one specific embodiment, the starting catalyst composition is reducedwith a solution of an alkali metal borohydride, dried and then heated(preferably in an oxidizing gas atmosphere) at a temperature in therange of about 300° C. to about 700° C. for at least about 10 minutes.In another specific embodiment, the starting catalyst composition isreduced with a solution of hydrazine and dried, and, optionally,thereafter heated (preferably in an oxidizing gas atmosphere) at atemperature in the range of about 300° C. to about 700° C. for at leastabout 10 minutes. In a further specific embodiment, the startingcatalyst composition is reduced with a solution containing formaldehydeand/or alkali metal fluoride and at least one alkali metal hydroxide anddried and, preferably, thereafter heated (preferably in an oxidizing gasatmosphere) at a temperature in the range of about 300° C. to about 700°C. for at least about 10 minutes. In still another specific embodiment,the starting catalyst composition is reduced with a solution containingdextrose, and at least one alkali metal hydroxide, dried and thereafterheated (preferably in an oxidizing gas atmosphere) at a temperature ofabout 300° C. to about 700° C. for at least about 10 minutes.

Also in accordance with this invention, catalyst compositions preparedby the above-described methods are provided.

Further in accordance with this invention, a process for selectivelyhydrogenating acetylene (preferably present in a small amount in anethylene-containing gas stream) with hydrogen gas to ethylene is carriedout with a catalyst prepared by one of the above described methods ofthis invention.

DETAILED DESCRIPTION OF THE INVENTION

The starting material (also referred to as "base catalyst") which iswet-reduced in accordance with this invention can be any supportedpalladium- and silver-containing catalyst composition. This catalystcomposition can be fresh (e.g., supplied by United Catalysts, Inc.,Louisville, Ky., under the product designation of "G-83C") or it can bea used and thereafter oxidatively regenerated catalyst composition. Thebase catalyst can contain any suitable solid support material.Preferably, the support material is selected from the group consistingof alumina (more preferably alpha-alumina), titania, zirconia andmixtures thereof. Presently most preferred is thepalladium/silver/alumina composition described in U.S. Pat. No.4,404,124, the disclosure of which is incorporated herein by reference.This skin-type catalyst generally contains about 0.01-1 (preferablyabout 0.01-0.2) weight-% palladium and about 0.01-10 (preferably about0.02-2) weight-% silver, at a Ag:Pd weight ratio of about 1:1 to about10:1, more preferably about 5:1 to about 8:1. The particle size of thesupported Pd/Ag base catalyst generally is about 1-10 mm, mostpreferably 2-6 mm. The supported Pd/Ag base catalyst particles can haveany suitable shape, and preferably are spheres or cylindrical pellets.Generally, the surface area of the supported Pd/Ag base catalyst(determined by the BET method employing N₂) is about 1-100 m² /g.

The above-described base catalyst particles are contacted with areduction composition (hereinafter referred to as "wet-reductioncomposition"), which contains at least one reducing agent and, if thereducing agent is not liquid, also at least one liquid component, inaccordance with this invention. Suitable reducing agents include (butare not limited to) alkali metal borohydrides (such as NaBH₄, KBH₄),hydrazine, aldehydes containing 1-6 carbon atoms per molecule(preferably formaldehyde), ketones containing 1-6 carbon atoms permolecule, carboxylic acids containing 1-6 carbon atoms per molecule(preferably, formic acid, ascorbic acid), reducing sugars which containan aldehyde group or, alternatively, an α-hydroxyketone group(preferably dextrose), aluminum metal (preferably powder), zinc metal(preferably powder), and the like, and mixtures thereof. In the case ofthe above-listed aldehydes, ketones, carboxylic acids, reducing sugars,zinc metal, and aluminum metal, these reducing agents are most effectivewhen an alkaline agent (preferably an alkali metal hydroxide and/orfluoride) is also present in the wet-reduction composition. When thereducing agent is a solid (e.g., Al powder), a liquid substance whichfunctions as a solvent or, alternatively, as a dispersing medium is alsopresent in the wet-reduction composition. Water or a lower aliphaticalcohol (in particular methanol), or mixtures thereof can serve as thenon-reducing liquid component. Generally, the weight percentage of theat least one reducing agent in the wet reduction composition is in therange of about 0.5 to about 50 weight-%, but may be even higher.Preferably, the pH of the wet-reduction composition, in particular if itis aqueous, is about 8-14.

In a particularly preferred embodiment of this invention, thewet-reduction composition additionally contains at least one dissolvedcompound of at least one alkali metal (preferably selected from thegroup consisting of potassium, rubidium and cesium). Presently preferredalkali metal compounds are halides, hydroxides, carbonates,bicarbonates, nitrates, carboxylates (such as acetates, oxalates,citrates and the like). Fluorides and hydroxides of potassium, rubidiumand cesium are particularly preferred, especially when the reducingagent is an aldehyde or ketone or carboxylic acid or reducing sugar orzinc or aluminum. Generally, the concentration of the alkali metalcompound in the wet-reduction composition and the weight ratio of alkalimetal compound to the base catalyst composition are chosen so as toincorporate about 0.05-5 weight-% alkali metal (on an elemental basis)into the catalyst composition. It is also possible to carry out thecontacting of the catalyst composition with at least one dissolvedalkali metal compound before the wet-reduction or, alternatively, afterthe wet-reduction has occurred. However, the essentially simultaneouswet-reduction and treatment with at least one alkali metal compound, asdescribed above, is the presently preferred method. It is, of course,also possible (yet presently not preferred) to carry out the contactingwith the alkali metal compound(s) both before said wet-reduction andconcurrently with said wet-reduction, or both concurrently with saidwet-reduction and after said wet-reduction, or before, concurrently withand after said wet-reduction.

The contacting of the supported Pd/Ag base catalyst composition with thewet reduction composition can be carried out in any suitable manner. Ingeneral, the catalyst composition and the wet-reduction composition arecontacted (mixed) for a time period of at least about 1 second,preferably about 10 seconds to about 10 hours, at a relatively lowtemperature of about 10°-60° C. More preferably, the time period isabout 0.02 to about 2 hours, and the temperature is in the range ofabout 20° C. to about 50° C. The pressure during the wet-reduction stepis approximately atmospheric (about 0 psig). This contacting step can becarried out as a batch-type operation (mixing or soaking) orcontinuously (e.g., by using a mixing screw or a static mixer equippedwith internal baffles or by spraying the base catalyst composition whichis placed on a moving conveyer belt with the wet-reduction composition).

The wet-reduced catalyst composition is then substantially separatedfrom the wet-reduction composition by any conventional solid-liquidseparation technique, such as filtering (presently preferred), decantingof the liquid, centrifuging, and the like. Thereafter, the substantiallyseparated, wet-reduced catalyst composition is dried, generally for atime period of about 0.2-20 hours (preferably about 2-6 hours), at atemperature of about 50°-150° C. (preferably about 100°-130° C.). It ispreferred to then heat (calcine) the dried, wet-reduced catalystcomposition, generally for a time period of about 0.2-20 hours(preferably about 1-6 hours) at a temperature of about 300°-700° C.(preferably about 400°-600° C.). Both the drying step and the calciningstep can be carried out in an oxidizing (i.e., O₂ -containing) gasatmosphere or in an inert gas atmosphere (e.g., under N₂, He, Ar, andthe like), preferably in air. This calcining step is particularlypreferred when an alkali metal hydroxide has been used as the reducingagent.

The thus-prepared catalyst composition which has been dried and,optionally, calcined can then be employed in a process for hydrogenatingacetylene to primarily ethylene. Optionally, the catalyst is firstcontacted, prior to the acetylene hydrogenation, with hydrogen gas orwith a gaseous hydrocarbon generally at a temperature in the range ofabout 30° C. to about 100° C., for a time period of about 4 to about 20hours. During this contacting with H₂ or hydrocarbon(s) before theselective acetylene hydrogenation commences, palladium and silvercompounds (primarily oxides) which may be present in the wet-reducedcatalyst composition after the drying step and the optional calciningstep (described above) are substantially reduced to palladium and silvermetal. When this optional reducing step is not carried out, the hydrogengas present in the reaction mixture accomplishes this reduction ofoxides of Pd and Ag during the initial phase of the acetylenehydrogenation reaction of this invention.

The selective hydrogenation process of this invention is carried out bycontacting (a) a feed gas which comprises acetylene, preferably anethylene stream containing acetylene as an impurity (generally at alevel of about 1 ppm to about 50,000 ppm C₂ H₂) and (b) hydrogen gaswith (c) the catalyst composition(s) of the present invention. In orderto best attain substantially complete removal of the acetylene, thereshould be at least one mole of hydrogen for each mole of acetylenepresent. Gases (a) and (b) are generally premixed before their contactwith the catalyst composition (c). It is within the scope of thisinvention to have additional gases (such as methane, ethane, propane,propene, butane, butenes, carbon monoxide, hydrogen sulfide) present inthe feed gas, as long as they do not significantly interfere with theselective hydrogenation of acetylene to ethylene. Generally, CO and H₂ Sare present in trace amounts (preferably less than about 0.5 weightpercent CO and less than about 50 ppm H₂ S).

The temperature necessary for the selective hydrogenation of acetyleneto ethylene depends largely upon the activity of the catalysts and theextent of acetylene removal desired. Generally, temperatures in therange of about 0° C. to about 150° C. are used. Any suitable reactionpressure can be employed. Generally, the total pressure is in the rangeof about 100 to about 1,000 pounds per square inch gauge (psig). The gashourly space velocity (GHSV) can also vary over a wide range. Typically,the space velocity will be in the range of about 1,000 to about 10,000m³ of feed per m³ of catalyst per hour, more preferably about 2,000 toabout 8,000 m³ /m³ /hour.

Regeneration of the catalyst composition can be accomplished by heatingthe catalyst composition in air (at a temperature which preferably doesnot exceed about 700 ° C.) to burn off any organic matter and/or charthat has been accumulated on the catalyst composition. Optionally, theoxidatively regenerated composition is reduced with H₂ or a suitablehydrocarbon (as has been described above) before its redeployment in theselective hydrogenation of acetylene. It is also within the scope ofthis invention to treat the oxidatively regenerated catalyst compositionwith a wet-reduction composition, followed by a drying step and anoptional calcining step (i.e., in accordance with the method of thisinvention) before the catalyst is redeployed in the selectivehydrogenation of acetylene (either directly or after reduction with H₂or a suitable hydrocarbon, as has been described above).

The following examples are presented to further illustrate thisinvention and are not to be construed as unduly limiting its scope.

Example I

This example illustrates the reduction of supported palladium catalystswith a dissolved alkali metal borohydride and the use of this"wet-reduced" catalyst for the selective hydrogenation of acetylene toethylene.

Catalyst A1 (Control) was a commercial Pd/Al₂ O₃ catalyst whichcontained 0.018 weight-% Pd and about 99 weight-% alumina. It had asurface area (determined by the BET method employing N₂) of 3-5 m² /g,and had been provided by United Catalysts Inc. (UCI), Louisville, Ky.under the product designation "G-83A".

Catalyst A2 (Control) was prepared by soaking 20 cc of Catalyst A1 in150 cc distilled water (which had been deaerated by bubbling N₂ gasthrough), adding 0.130 grams of NaBH₄ to the catalyst/water mixture, andstirring the entire mixture (containing the catalyst, water and NaBH₄)for about 1 hour at room temperature. Then the excess liquid was drainedand the soaked catalyst was washed several times with deaerated water.The wet catalyst was heated in a hydrogen gas atmosphere at 90° C. for 2hours, and allowed to cool to room temperature under flowing H₂.

Catalyst A3 (Control) was prepared by soaking 23.65 grams of Catalyst A1in 100 cc of methanol (which had been deaerated with flowing N₂), addingabout 1 gram of solid NaBH₄ to the catalyst/methanol mixture, stirringthe entire mixture (containing the catalyst, methanol and NaBH₄) forabout 1 hour at room temperature, draining excess liquid, washing thesoaked catalyst three times with fresh methanol, drying the washedcatalyst at 180° F. (82° C.) overnight, and calcining the dried catalystin 370° C. for about 4 hours.

Catalyst B1 (Control) was a commercial Pd/Ag/Al₂ O₃ catalyst whichcontained 0.023 weight-% Pd, 0.065 weight-% Ag and about 99 weight-%alumina. It had a BET/N₂ surface area of 3-5 m² /g, had been preparedsubstantially in accordance with the method described in U.S. Pat. No.4,404,124 (column 4, lines 32-45), and had been provided by UCI(identified above) under the product designation "G-83C".

Catalyst B2 (Invention) was prepared by soaking about 20 cc of CatalystB1 in 150 cc of deaerated methanol, adding about 1.0 gram of solid NaBH₄to the catalyst/methanol mixture, stirring the entire mixture(containing the catalyst, methanol and NaBH₄) for about 90 minutes atroom temperature, draining excess liquid, washing the soaked catalystthree times with deaerated methanol, and drying the washed catalystunder vacuum conditions.

Catalyst B3 (Invention) was prepared by soaking 20 cc of Catalyst B1 in100 cc deaerated methanol, adding about 1.0 gram of solid NaBH₄ to thecatalyst/methanol mixture, stirring the entire mixture (containing thecatalyst, methanol and NaBH₄) for about 1 hour at room temperature,draining excess liquid, washing the soaked catalyst three times withdeaerated methanol, drying the washed catalyst overnight at 180° F.(82°), and calcining the dry catalyst at 370° C. in air for about 3hours.

Catalyst B4 (Invention) was prepared by calcining Catalyst B2 after ithad been tested for its acetylene hydrogenation performance at 370° C.in air for about 4 hours.

About 20 cc of each of the above-described catalysts was placed into astainless steel reactor tube having a 0.5 inch inner diameter and alength of about 18 inches. Each catalyst was treated with flowinghydrogen gas under a pressure of 200 psig, at a temperature of about110°-130° F., for about 16 hours. Thereafter, the reactor tube wascooled to about 110° F., and a hydrocarbon-containing feed gascontaining 1.98 weight-% hydrogen, 21.40 weight-% methane, 21.18weight-% ethane, 55.09 weight-% ethylene, 0.35 weight-% acetylene and0.03 weight-% carbon monoxide was introduced into the reactor tube at arate of about 900 cc/minute. The reactor temperature was graduallyincreased to the desired reaction temperature, and samples of the formedproduct were analyzed by means of a gas chromatograph at various timeintervals.

Two key test results are summarized in Table I: T₁, which is the"cleanup" temperature at which acetylene is substantially hydrogenatedto ethylene (so as to obtain a product containing less than about 10 ppm(ppm=parts per million parts by weight) of acetylene; and T₂ , which isthe "runaway" temperature at which a substantial portion of ethylene isconverted to ethane (exothermic "runaway" reaction) and theethylene/ethane molar ratio in the product has decreased to about 2.3(from about 2.8 attained at lower temperatures). The higher thetemperature difference (T₁ -T₂) attained with a particular catalyst, themore satisfactorily will this catalyst perform as a selective acetylenehydrogenation catalyst. Test results are summarized in Table I.

                  TABLE I    ______________________________________    Run    Catalyst        T.sub.1 (°F.)                                    T.sub.2 (°F.)                                           T.sub.2 -T.sub.1    ______________________________________    1      A1 (Pd/Al.sub.2 O.sub.3, not wet-                           114      143    29    (Control)           reduced)    2      A2 (Pd/Al.sub.2 O.sub.3, wet-                           116      145    29    (Control)           reduced with NaBH.sub.4)    3      A3 (Pd/Al.sub.2 O.sub.3, wet-                           133      164    31    (Control)           reduced with NaBH.sub.4,           and calcined)    4      B1 (Pd/Ag/Al.sub.2 O.sub.3, not                           120      155    35    (Control)           wet-reduced)    5      B2 (Pd/Ag/Al.sub.2 O.sub.3, wet-                           192      232    40    (Inven-           reduced with NaBH.sub.4)    tion)    6      B3 (Pd/Ag/Al.sub.2 O.sub.3, wet-                           134      183    49    (Inven-           reduced with NaBH.sub.4,    tion)  and calcined)    7      B4 (Pd/Ag/Al.sub.2 O.sub.3, wet-                           141      190    49    (Inven-           reduced with NaBH.sub.4,    tion)  and calcined)    ______________________________________

Test results in Table I show that wet-reduction of a Pd/Al₂ O₃ catalystwith dissolved sodium borohydride, either with or without subsequentcalcination, had no significant effect on T₂ -T₁ (i.e., the temperaturedifference defined above). On the other hand, the wet-reduction of thePd/Ag/Al₂ O₃ catalyst resulted in an enhanced temperature difference, inparticular when calcination in air was carried out after thewet-reduction. The presence of traces of formed C₄ hydrocarbons (about0.02 weight-%) in the hydrogenation product was observed.

Example II

This example illustrates the wet-reduction of a supportedpalladium/silver catalyst with dissolved hydrazine in accordance withthis invention.

Catalyst C1 (Control) was substantially the same as Catalyst B1described in Example I.

Catalyst C2 (Invention) was prepared by soaking 23.3 grams of CatalystC1 with a solution of 5 cc N₂ H₄.H₂ O in 45 cc of distilled water forabout 1 hour. The excess liquid was poured off, and the wet catalyst washeated in a forced air convection oven at 125° C. for about 4 hours.

Both catalysts were tested for their selective acetylene hydrogenationactivity, substantially in accordance with the procedure described inExample I, except that the hydrocarbon-containing feed contained about33.2 weight-% methane, about 0.08 weight-% ethane, about 63.1 weight-%ethylene, about 0.35 weight-% acetylene about 0.05 weight-% carbonmonoxide, and about 3.2 weight-% H₂. Test results are summarized inTable II.

                                      TABLE II    __________________________________________________________________________                                   ppm Ethane    Run    Catalyst  T.sub.1 (°F.)                          T.sub.2 (°F.)                               T.sub.2 -T.sub.1                                   Formed at T.sub.1    __________________________________________________________________________    8      C1 (Pd/Ag/Al.sub.2 O.sub.3,                     107  145  38  2970    (Control)           not wet-reduced    9      C2 (Pd/Ag/Al.sub.2 O.sub.3,                     123  166  43  2290    (Invention)           wet-reduced with           N.sub.2 H.sub.4)    __________________________________________________________________________

Test data in Table II show that wet-reduction of a Pd/Ag/Al₂ O₃ catalystwas beneficial in terms of increasing the temperature difference T₂ -T₁(defined in Example I). The presence of trace amounts of formed butenes(0.03-0.05 weight-%) the hydrogenation product was observed.

Table II also presents test data on the amount (in ppm by weight) ofethane produced at the "cleanup" temperature, T₁, which is a measure ofthe selectivity to ethylene exhibited by a particular catalyst at the"cleanup" temperature (defined in Example I). These test data wereobtained by subtracting the amount of ethane present in the feed fromthe amount of ethane present in the hydrogenation product produced atT₁. Table II clearly shows that less of the undesirable ethane wasformed with the wet-reduced catalyst, C₂ (as compared with the untreatedCatalyst C₁). Consequently, more desirable ethylene was produced withthe invention Catalyst C₂ (as compared with the untreated Catalyst C₁).

Example III

This example illustrates the wet reduction of a Pd/Ag/Al₂ O₃ withformaldehyde in the presence of an alkali metal hydroxide, in accordancewith this invention.

Catalyst D1 (Invention) was prepared by mixing 20 cc of control CatalystB1 (described in Example I) with about 110 cc of a solution whichcontained about 38 weight-% formaldehyde, about 12 weight-% methanol andabout 50 weight-% water, and 1.13 grams of NaOH. The entire mixture wasstirred at room temperature for about 2 hours, excess liquid was pouredoff, the soaked catalyst was rinsed three times with fresh methanol, thewashed catalyst was dried overnight at about 88° C., and the driedcatalyst was then calcined in air at 370° C. for 4.5 hours.

Catalyst D2 (Invention) was prepared by mixing 20 cc of control CatalystC1 (described in Example II) with 110 cc of a solution, which containedabout 37 weight-% formaldehyde, about 17 weight-% methanol and about 46weight-% water, and 1.18 grams of NaOH. The entire mixture was stirredat room temperature for about 2 hours. Excess liquid was poured off. Thewet catalyst was rinsed three time with 150 cc of fresh methanol, anddried under H₂ at 60°-100° C. for several hours.

Catalyst D3 (Invention) was prepared by mixing 20 cc of Catalyst C1 with75 cc of a solution, which contained about 37 weight-% formaldehyde,about 17 weight-% methanol and about 46 weight-% water, and about 0.6gram of KOH. The entire mixture was stirred at room temperature forabout 40 minutes, excess liquid was poured off, the wet catalyst wasdried at about 88° C., and was then calcined at 200° C. for severalhours.

The thus-prepared catalysts were tested for the selective acetylenehydrogenation activity, substantially in accordance with the proceduredescribed in Example I. The hydrocarbon-containing feed had essentiallythe same composition as the feed employed in the tests of Example I.Test results for the three catalysts described in this Example aresummarized in Table III. Also included in this table are the testresults from control Run 4 (see Table I) employing control Catalyst B1(for comparison with Run 10 employing invention Catalyst D1). Resultsfrom another control run (Run 11 employing Catalyst C1) is listed forcomparison with invention Catalysts D2 and D3 (Runs 12 and 13).

                  TABLE III    ______________________________________    Run     Catalyst       T.sub.1 (°F.)                                    T.sub.2 (°F.)                                           T.sub.2 -T.sub.1    ______________________________________    4       B1 (Pd/Ag/Al.sub.2 O.sub.3,                           120      155    35    (Control)            not wet-reduced)    10      D1 (Pd/Ag/Al.sub.2 O.sub.3,                           125      171    46    (Invention)            wet-reduced with            CH.sub.2 O in presence of            NaOH, and            calcined)    11      C1 (Pd/Ag/Al.sub.2 O.sub.3,                           134      167    33    (Control)            not wet-reduced)    12      D2 (Pd/Ag/Al.sub.2 O.sub.3,                           129      182    53    (Invention)            wet-reduced with            CH.sub.2 O in presence of            NaOH),    13      D3 (Pd/Ag/Al.sub.2 O.sub.3,                           143      209    66    (Invention)            wet-reduced with            CH.sub.2 O in presence of            KOH, and calcined)    ______________________________________

Test results in Table III clearly show that wet-reduction of a Pd/Ag/Al₂O₃ catalyst with formaldehyde in the presence of an alkali metalhydroxide (with or without subsequent calcination) was quite effectivein increasing the temperature difference T₂ -T₁ (defined in Example I).The presence of trace amounts of C₄ hydrocarbons (0.01-0.02 weight-%) inthe hydrogenation product was observed.

Example IV

This example illustrates the wet-reduction of supported palladiumcatalysts with formaldehyde in the presence of alkali metal compoundsand the performance of these materials as catalysts in the selectivehydrogenation of acetylene.

Catalyst E1 (Control) was a commercial "G-83A" Pd/Al₂ O₃ catalyst,described in Example I (Catalyst A1).

Catalyst E2 (Control) was prepared by soaking, with occasional stirring,30 grams of Catalyst E1 with a solution of 0.42 gram of 88 weight-% KOHpellets in 26 grams of distilled water for about 1 hour at roomtemperature, followed by draining excess solution, drying for 16 hoursat 125° C., and calcining in air for 2 hours at 1000° F. (538° C.).

Catalyst E3 (Control) was prepared in accordance with the procedure forCatalyst E2, except that Catalyst E1 was soaked with a mixture of 0.42gram of 88% KOH pellets and 26 grams of a commercially availableformaldehyde solution containing 38 weight-% CH₂ O, 12 weight-% CH₃ OHand 50 weight-% H₂ O.

Catalyst E4 (Control) was a commercial "G-83C" Pd/Ag/Al₂ O₃ catalystdescribed in Example I (Catalyst B1).

Catalyst E5 (Control) was prepared by soaking, with occasional stirring,30 grams of Catalyst E4 with a solution of 0.48 gram of 88% KOH pelletsin 26 grams of water for about 1 hour at room temperature, followed bydraining excess solution, drying for 8 hours at 125° C., and calciningin air for 2 hours at 1000° F. (538° C.).

Catalyst E6 (Invention) was prepared in accordance with the procedurefor Catalyst E5, except that Catalyst E4 was soaked with a mixture of0.48 gram of 88% KOH pellets and 26 grams of the above-describedformaldehyde solution.

Catalysts E1 through E6 were tested substantially in accordance with theprocedure described in Example I. Each catalyst had been preheated for 1hour with flowing hydrogen gas at 100° F./200 psig and then for 1 hourwith the hydrocarbon-containing feed (containing methane, ethane,ethylene, acetylene, carbon monoxide and hydrogen; similar to the feeddescribed in Example II) at 100° F./200 psig. Thereafter, thetemperature was gradually raised while the feed passed through thereactor. Test results are summarized in Table IV.

                                      TABLE IV    __________________________________________________________________________                                    ppm Ethane    Run    Catalyst   T.sub.1 (°F.)                           T.sub.2 (°F.)                                T.sub.2 -T.sub.1                                    Formed at T.sub.1    __________________________________________________________________________    14     E1 (Pd/Al.sub.2 O.sub.3, not                      115  134  19  8850    (Control)           wet-reduced)    15     E2 (Pd/Al.sub.2 O.sub.3, not                      128  181  53  3400    (Control)           wet-reduced but           treated with KOH,           calcined)    16     E3 (Pd/Al.sub.2 O.sub.3, wet-                      135  186  51  3440    (Control)           reduced with CH.sub.2 O           in presence of KOH,           calcined)    17     E4 (Pd/Ag/Al.sub.2 O.sub.3,                      110  158  48  1830    (Control)           not wet-reduced)    18     E5 Pd/Ag/Al.sub.2 O.sub.3, not                      123  190  67  1900    (Control)           wet-reduced but           treated with KOH,           calcined)    19     E6 Pd/Ag/Al.sub.2 O.sub.3,                      133  211  78   840    (Invention)           wet-reduced with           CH.sub.2 O in presence of           KOH, calcined)    __________________________________________________________________________

Test data in Table IV show that wet-reduction of a Pd/Al₂ O₃ catalyst(without Ag) with formaldehyde in the presence of an alkali metalcompound (KOH) showed no improvement (in terms of T₂ -T₁ and of ethaneformed at T₁) over treatment with KOH alone (compare Run 16 with Run15). In contrast, wet-reduction of a Pd/Ag/Al₂ O₃ catalyst withformaldehyde in the presence of KOH in accordance with this inventionresulted in a significant increase in T₂ -T₁ and also in a significantdecrease of ethane formed at T₁ (compare Run 19 with Run 18).

Example V

This example further illustrates a particularly preferred feature ofthis invention: wet-reduction of Pd/Ag/Al₂ O₃ catalysts withformaldehyde in the presence of alkali metal compounds.

Catalyst F1 (Control) was a commercial "G-83C" Pd/Ag/Al₂ O₃ catalyst(essentially the same as Catalyst B1, Example I).

Catalyst F2 (Control) was a "G-83C" Pd/Ag/Al₂ O₃ catalyst (definedabove) which had been used for the selective hydrogenation of acetyleneto ethylene in a Texas refinery of Phillips Petroleum Company, and hadthen been regenerated by heating it in air to a temperature of 1000° F.within a time period of 3 hours, followed by calcining in air at thattemperature for 4 hours. Thereafter, the calcined, used catalyst wascooled to room temperature.

Catalyst F3 (Invention) was prepared as follows 0.51 gram of 88 weight-%KOH pellets (equivalent to 0.008 mole KOH) was added to 30 grams of a37-38 weight-% formaldehyde solution in methanol. The formed solutionwas stirred for about one minute, 30 grams of control Catalyst F2(regenerated "G-83C ") were added to the solution, and the obtainedmixture was kept at room temperature for 1 hour, with occasionalstirring. Then excess liquid was decanted, the soaked tablets were driedin air at 125° C. for 5 hours, and the dried tablets were calcined inair at 538° C. for 2 hours.

Catalyst F4 (Invention) was prepared essentially in accordance with theabove-described procedure for Catalyst F3 except that 0.97 gram of 99%pure RbOH.H₂ O (0.008 mole RbOH) was used (in lieu of 0.008 mole KOH).

Catalyst F5 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F3 except that 0.008 mole CsOH was used (in lieuof 0.008 mole KOH).

Catalyst: F6 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F3 except that Catalyst F1 (fresh "G-83C") wasused as the starting material (in lieu of Catalyst F2).

Catalyst F7 (Invention) was prepared essentially in accordance withCatalyst F4 except that Catalyst F1 was used as the starting material(in lieu of Catalyst F2).

Catalyst F8 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F5 except that Catalyst F1 was used as thestarting material (in lieu of Catalyst F2).

Catalyst F9 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F6 except that 0.002 mole KOH was employed (inlieu of 0.008 mole KOH).

Catalyst F10 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F7 except that 0.002 mole RbOH was employed (inlieu of 0.008 mole RbOH).

Catalyst F11 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F8 except that 0.002 mole CsOH was employed (inlieu of 0.008 mole CsOH).

Catalyst F12 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F6 except that 0.032 mole of KOH was employed (inlieu of 0.008 mole of KOH).

Catalyst F13 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F12 except that 0.032 mole of NaOH (in lieu ofKOH) was employed.

Catalyst F14 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F3 except that 0.005 mole of KOH was employed (inlieu of 0.008 mole KOH).

Catalyst F15 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F3 except that 0.0275 mole of KOH was employed(in lieu of 0.008 mole KOH), the time of contact with the formaldehydesolution was only about 0.5 hour (in lieu of 1 hour), and theformaldehyde concentration was only about 18 weight-% (in lieu of37-38%).

Catalyst F16 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F3 except that 0.015 mole of KOH was employed (inlieu of 0.008 mole KOH), and the formaldehyde concentration was onlyabout 1 weight-% (in lieu of 37-38%).

Catalyst F17 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F3 except that 0.050 mole of KOH was employed (inlieu of 0.008 mole of KOH).

Catalyst F18 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F6 except that 0.05 mole of KF was employed (inlieu of 0.008 mole of KOH).

Catalyst F19 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F18, except that 0.007 mole of KF and 0.001 moleof KOH (in lieu of 0.008 mole KF) was employed.

Catalysts F1 through F19 were tested substantially in accordance withthe procedure described in Example I, except that a hydrocarbon feedwhich was essentially the same as that described in Example II wasemployed. Test results are summarized in Table V.

                                      TABLE V    __________________________________________________________________________                                     ppm Ethane    Run    Catalyst    T.sub.1 (°F.)                            T.sub.2 (°F.)                                 T.sub.2 -T.sub.1                                     Formed at T.sub.1    __________________________________________________________________________    20     F1 (fresh   125  169  44  4320    (Control)           (Pd/Ag/Al.sub.2 O.sub.3, not wet-           reduced)    21     F2 (regenerated                       139  176  37  3950    (Control)           (Pd/Ag/Al.sub.2 O.sub.3, not wet-           reduced)    22     F3 (Pd/Ag/Al.sub.2 O.sub.3, wet-                       141  230  89  760    (Invention)           reduced with CH.sub.2 O in           presence of KOH)    23     F4 (Pd/Ag/Al.sub.2 O.sub.3, wet-                       159  235  76  1650    (Invention)           reduced with CH.sub.2 O in           presence of RbOH)    24     F5 (Pd/Ag/Al.sub.2 O.sub.3, wet-                       177  290  113 700    (Invention)           reduced with CH.sub.2 O in           presence of CsOH)    25     F6 (Pd/Ag/Al.sub.2 O.sub.3, wet-                       147  231  84  750    (Invention)           reduced with CH.sub.2 O in           presence of KOH)    26     F7 (Pd/Ag/Al.sub.2 O.sub.3, wet-                       158  260  102 572    (Invention)           reduced with CH.sub.2 O in           presence of RbOH)    27     F8 (Pd/Ag/Al.sub.2 O.sub.3, wet-                       188  289  101 800    (Invention)           reduced with CH.sub.2 O in           presence of CsOH)    28     F9 (Pd/Ag/Al.sub.2 O.sub.3, wet-                       146  218  72  1170    (Invention)           reduced with CH.sub.2 O in           presence of KOH)    29     F10 (Pd/Ag/Al.sub.2 O.sub.3,                       147  227  80  840    (Invention)           wet-reduced with CH.sub.2 O           in presence of RbOH)    30     F11 (Pd/Ag/Al.sub.2 O.sub.3,                       158  237  79  670    (Invention)           wet-reduced with           CH.sub.2 O in presence of           CsOH)    31     F12 (Pd/Ag/Al.sub.2 O.sub.3,                       146  266  120 579    (Invention)           wet-reduced with CH.sub.2 O           in presence of KOH)    32     F13 (Pd/Ag/Al.sub.2 O.sub.3,                       149  203  54  2350    (Invention)           wet-reduced with CH.sub.2 O           in presence of NaOH)    33A    F14 (Pd/Ag/Al.sub.2 O.sub.3,                       145  212  67  1250    (Invention)           wet-reduced with CH.sub.2 O           in presence of KOH)    33B    F14 (Pd/Ag/Al.sub.2 O.sub.3,                       147  209  62  1200    (Invention)           wet-reduced with CH.sub.2 O           in presence of KOH)    34     F15 (Pd/Ag/Al.sub.2 O.sub.3,                       149  237  88  540    (Invention)           wet-reduced with CH.sub.2 O           in presence of KOH)    35     F16 (Pd/Ag/Al.sub.2 O.sub.3,                       141  241  100 350    (Invention)           wet-reduced with           CH.sub.2 O in presence of           KOH)    36     F17 (Pd/Ag/Al.sub.2 O.sub.3,                       151  238  87  480    (Invention)           wet-reduced with CH.sub.2 O           in presence of KOH)    37     F18 (Pd/Ag/Al.sub.2 O.sub.3,                       161  294  133 500    (Invention)           wet-reduced with           CH.sub.2 O, in presence           of KF)    38     F19 (Pd/Ag/Al.sub.2 O.sub.3,                       136  233  97  780    (Invention)           wet-reduced with           CH.sub.2 O, in presence of           KF + KOH)    __________________________________________________________________________

Test data in Table V clearly show that the reduction of fresh or ofused, regenerated Pd/Ag/Al₂ O₃ catalysts with formaldehyde in thepresence of an alkali metal compound consistently resulted in higher T₂-T₁ values and in lower amounts of ethane formed at the cleanuptemperature, T₁. Most effective alkali metal compounds were KOH, KF,RbOH and CsOH. Generally, the use of formaldehyde solutions containing0.002-0.050 mole KOH or RbOH or CsOH or KF were most effective.Additional test data (not included in Table V) indicate that the amountsof ethane produced at temperatures which were 10° F., 20° F. and 30° F.,respectively, above the particular "cleanup" temperature wereconsistently lower in invention runs 22-38 than in control runs 20 and21. Thus, the wet-reduced, alkali-metal-promoted Pd/Ag/Al₂ O₃ catalystswere more selective to ethylene (rather than ethane) than the untreatedcatalysts, at comparable conversions of acetylene.

Example VI

This example illustrates wet-reduction of Pd/Ag/Al₂ O₃ catalysts with adissolved reducing agent other than formaldehyde, in the presence of analkali metal compound (KOH).

Catalyst G1 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F3 (Example V) except an aqueous solutioncontaining 0.03 mole of formic acid and 28.5 grams of water was used asthe reducing agent (in lieu of formaldehyde/methanol/water).

Catalyst G2 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F3 (Example V) except that an aqueous solutioncontaining 0.03 mole of ascorbic acid (Vitamin C) and 24.7 grams ofwater was used as the reducing agent (in lieu offormaldehyde/methanol/water).

Catalyst G3 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F3 (Example V) except that an aqueous solutioncontaining 0.03 mole of hydrazine hydrate and about 29.0 grams of waterwas used as the reducing agent (in lieu of formaldehyde/methanol/water).

Catalyst G4 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F3 (Example V) except that an aqueous solutioncontaining 0.03 mole of dextrose and 24.6 grams of water was used as thereducing agent (in lieu of formaldehyde/methanol/water).

Catalyst G5 (Invention) was prepared essentially in accordance with theprocedure for Catalyst G4 except that 0.003 mole dextrose (in lieu of0.03 mole dextrose) and 0.025 mole KOH was employed (in lieu of 0.005mole KOH).

Catalyst G6 (Invention) was prepared essentially in accordance with theprocedure for Catalyst G5 except that 0.015 mole KOH was employed (inlieu of 0.025 mole KOH).

Catalyst G7 (Invention) was prepared essentially in accordance with theprocedure for Catalyst F3 (Example V) except that a mixture of 0.38grams of aluminum metal powder, 0.96 grams of KOH and 20 grams of waterwas used as the reducing agent (in lieu of formaldehyde/methanol/water).

Catalysts G1-G7 were tested substantially in accordance with theprocedure used in Example V. Test results are summarized in Table VI.

                                      TABLE VI    __________________________________________________________________________                                    ppm Ethane    Run    Catalyst   T1 (°F.)                           T2 (°F.)                                T.sub.2 -T.sub.1                                    Formed at T.sub.1    __________________________________________________________________________    21     F2 (regenerated                      139  176  37  3950    (Control)           Pd/Ag/Al.sub.2 O.sub.3, not           wet-reduced)    39     G1 (Pd/Ag/Al.sub.2 O.sub.3,                      137  207  70  894    (Invention)           wet-reduced with           formic acid, in           presence of KOH)    40     G2 (Pd/Ag/Al.sub.2 O.sub.3,                      132  216  84  630    (Invention)           wet-reduced           ascorbic acid, in           presence of KOH)    41     G3 (Pd/Ag/Al.sub.2 O.sub.3,                      138  229  91  550    (Invention)           wet-reduced with           hydrazine, in           presence of KOH)    42     G4 (Pd/Ag/Al.sub.2 O.sub.3,                      143  210  67  1300    (Invention)           wet-reduced with           dextrose, in presence           of KOH)    43     G5 (Pd/Ag/Al.sub.2 O.sub.3,                      144  265  121 350    (Invention)           wet-reduced with           dextrose, in presence           of KOH)    44     G6 (Pd/Ag/Al.sub.2 O.sub.3,                      141  241  100 750    (Invention)           wet-reduced with           dextrose, in presence           of KOH)    45     G7 (Pd/Ag/Al.sub.2 O.sub.3,                      181  298  117 --    (Invention)           wet-reduced with Al,           in presence of KOH)    __________________________________________________________________________

A comparison of the test data in Table VI with those in Table V indicatethat the four reducing agents employed in this Example were essentiallyas effective as formaldehyde, in the presence of potassium hydroxide.

Example VII

This example illustrates that the wet-reduction of supported Pd/Agcatalysts containing inorganic supports other than alumina (namelytitania and zirconia) was also quite effective in increasing T₂ -T₁values in the selective hydrogenation of acetylene, in accordance withthis invention.

Catalyst H1 (Control) was a Pd/Ag/TiO₂ catalyst which was prepared byimpregnating 1/8 inch, sulfur-free titania pellets with an aqueouspalladium(II) nitrate solution (so as to provide a level of about 0.02weight-% Pd on the TiO₂ pellets), drying the Pd/TiO₂ particles,calcining them for 8 hours at 400° C. in air, impregnating the calcinedPd/TiO₂ particle with an aqueous silver nitrate solution (so as toprovide a level of about 0.1 weight-% Ag in the catalyst), drying thePd/Ag/TiO₂ catalyst particles, and then calcining them for 8 hours at400° C. in air.

Catalyst H2 (Invention) was prepared by soaking Catalyst H1 with amixture of 26.0 grams of the commercial formaldehyde/methanol/watersolution described in Example IV (see preparation of Catalyst E3) and0.41 grams of 88 weight-% KOH pellets, followed by draining excesssolution, drying (at 125° C., for 5 hours) and calcining in air for 2hours at 400° C.

Both catalysts were tested substantially in accordance with theprocedure described in Example I, except that a hydrocarbon-containingfeed similar to the one described in Example II was used. Test resultsare summarized in Table VII.

                                      TABLE VII    __________________________________________________________________________                                    ppm Ethane    Run    Catalyst   T1 (°F.)                           T2 (°F.)                                T2-T1                                    Formed at T.sub.1    __________________________________________________________________________    46     H1 (Pd/Ag/TiO.sub.2,                      127  167  40  3900    (Control)           not wet-reduced    47     H2 (Pd/Ag/TiO.sub.2,                      126  191  65  1100    (Invention)           wet-reduced with           CH.sub.2 O in presence           of KOH    __________________________________________________________________________

Test data in Table VII clearly show that wet-reduction of Pd/Ag/TiO₂ hadessentially the same beneficial effects as wet-reduction of Pd/Ag/Al₂ O₃(described in previous examples).

Additional preliminary tests (not described in detail herein) indicatethat the use of a Pd/Ag/ZrO₂ catalyst (which had not been wet-reduced inthe selective hydrogenation of acetylene) was almost as effective (interms of T₂ -T₁ values) as Pd/Ag/Al₂ O₃ catalysts (also notwet-reduced). Based on these preliminary test results, it is concludedthat the wet-reduction of Pd/Ag/ZrO₂ catalysts, with or without thepresence of alkali metal compounds, in accordance with this inventionwill produce catalysts exhibiting enhanced T₂ -T₂ values and higherselectivities to ethylene (as compared with the untreated catalysts).Example VIII

This example illustrates a method of preparing an effective acetylenehydrogenation catalyst by wet-reduction and subsequent promotion withpotassium fluoride.

Catalyst I (Invention) was prepared by soaking 23.3 grams of Catalyst B1 ("G-83C", described in Example I) in 30 cc of a 37 weight-%formaldehyde solution (described in Example III for Catalyst D2), addingabout 0.5 gram solid KOH to the catalyst/solution mixture, stirring thismixture for 30 minutes at room temperature, adding again about 0.5 gramof solid KOH, and stirring the obtained mixture again for about 30minutes at room temperature. Thereafter, excess liquid was drained, andthe soaked, wet-reduced mixture was washed twice with fresh methanol andthen twice with distilled water (so as to remove substantially all KOHwhich had been incorporated into the catalyst). The washed, wet-reducedcatalyst was dried overnight at 180° F., and then soaked with a solutionof 0.355 gram anhydrous potassium fluoride in 7.58 grams of water. TheKF-impregnated catalyst was dried overnight at 180° F. and calcined for1.5 hours in air at 370° C.

The thus obtained catalyst, which contained about 1 weight-% K (asfluoride), was tested for its acetylene hydrogenation activitysubstantially in accordance with the procedure described in Example I.Result: T₁ was 154° F., T₂ was 245° F., and T₂ -T₁ was 91° F.

Thereafter, the catalyst was exposed to the feed at "runaway"conditions, i.e., at a temperature in excess of 245° F., for about 22minutes, so as to simulate possible damage to the catalyst by a"runaway" reaction in a plant operation. Then the catalyst was testedagain (after lowering the temperature of the reactor while H₂ gas passedthrough). Result: T₁ was 161° F., T₂ was 259° F., and T₂ -T₁ was 98° F.This result clearly indicates that no adverse effect on the KF-promoted,wet-reduced Pd/Ag/Al₂ O₃ catalyst had occurred by this exposure to"runaway" conditions.

Reasonable variations, modifications and adaptations for various usagesand conditions can be made within the scope of the disclosure and theappended claims, without departing from the scope of this invention.

That which is claimed is:
 1. In a process for the selectivehydrogenation of acetylene to ethylene which comprises mixing (a) a feedgas comprising acetylene and (b) hydrogen gas and contacting the mixtureof (a) and (b) with (c) an effective catalyst composition, theimprovement which comprises employing a catalyst composition which hasbeen prepared by a method consisting essentially of the steps of:(1)contacting a solid composition comprising palladium, silver and aninorganic support material with a liquid reduction compositioncomprising (i) at least one reducing agent selected from the groupconsisting of alkali metal borohydrides, hydrazine, aldehydes containing1-6 carbon atoms per molecule, ketones containing 1-6 carbon atoms permolecule, carboxylic acids containing 1-6 carbon atoms per molecule,aluminum metal and zinc metal, (ii) at least one non-reducing liquidcomponent, and (iii) at least one dissolved alkali metal compoundselected from the group consisting of alkali metal hydroxides and alkalimetal fluorides, at a temperature of up to about 60° C. for a timeperiod of at least about 1 second, so as to produce a wet-reduced solidcomposition; (2) substantially separating said wet-reduced solidcomposition produced in step (1) from said liquid reduction composition;and (3) drying the substantially separated, wet-reduced solidcomposition obtained in step (2).
 2. A process in accordance with claim1, wherein said inorganic support material is selected from the groupconsisting of alumina, titania, zirconia and mixtures thereof; said atleast one reducing agent is selected from the group consisting of sodiumborohydride, potassium borohydride, hydrazine, formaldehyde, formicacid, ascorbic acid and aluminum metal; said at least one non-reducingliquid component is selected from the group consisting of water,methanol and mixtures thereof; said at least one dissolved alkali metalcompound is selected from the group consisting of potassium hydroxide,potassium fluoride, rubidium hydroxide, rubidium fluoride, cesiumhydroxide and cesium fluoride; said temperature is about 10°-60° C.; andsaid time period is about 10 seconds to about 10 hours.
 3. A process inaccordance with claim 2, wherein said temperature in step (1) is about20°-50° C., said time period of step (1) is about 0.02-2 hours, and thepressure in step (1) is approximately atmospheric.
 4. A process inaccordance with claim 2 wherein said solid composition used in step (1)contains about 0.01-0.2 weight-% palladium, about 0.02-2 weight-%silver, and alumina as the inorganic support material.
 5. A process inaccordance with claim 4, wherein said at least one reducing agent usedin step (1) is selected from the group consisting of sodium borohydrideand potassium borohydride.
 6. A process in accordance with claim 4,wherein said at least one reducing agent is hydrazine, and said at leastone dissolved alkali metal compound is selected from the groupconsisting of potassium hydroxide, potassium fluoride, rubidiumhydroxide and cesium hydroxide.
 7. A process in accordance with claim 4,wherein said at least one reducing agent used in step (1) isformaldehyde, and said at least one dissolved alkali metal compound isselected from the group consisting of potassium hydroxide, potassiumfluoride, rubidium hydroxide and cesium hydroxide.
 8. A process inaccordance with claim 4, wherein said at least one reducing agent usedin step (1) is selected from the group consisting of formic acid andascorbic acid, and said at least one dissolved alkali metal compound isselected from the group consisting of potassium hydroxide, potassiumfluoride, rubidium hydroxide and cesium hydroxide.
 9. A process inaccordance with claim 4, wherein said at least one reducing agent usedin step (1) is aluminum metal powder, and said at least one dissolvedalkali metal compound is selected from the group consisting of potassiumhydroxide, potassium fluoride, rubidium hydroxide and cesium hydroxide.10. A process in accordance with claim 1, wherein the weight percentageof said at least one reducing agent in said liquid reduction compositionis employed in step (1) is about 0.5-50 weight-%.
 11. A process inaccordance with claim 1, wherein said feed gas comprises acetylene as animpurity in an ethylene stream, at an acetylene concentration of about1-50,000 ppm.
 12. A process in accordance with claim 1, wherein saidselective hydrogenation is carried out at a temperature of about 0° C.to about 150° C. and a pressure of about 100 to about 1,000 psig.
 13. Ina process for the selective hydrogenation of acetylene to ethylene whichcomprises mixing (a) a feed gas comprising acetylene and (b) hydrogengas and contacting the mixture of (a) and (b) with (c) an effectivecatalyst composition, the improvement which comprises employing acatalyst composition which has been prepared by a method consistingessentially of the steps of:(1) contacting (a) a solid compositioncomprising palladium, silver and an inorganic support material with (b)a liquid reduction composition comprising (i) at least one reducingagent selected from the group consisting of alkali metal borohydrides,hydrazine, aldehydes containing 1-6 carbon atoms per molecule, ketonescontaining 1-6 carbon atoms per molecule, carboxylic acids containing1-6 carbon atoms per molecule, sugars containing an aldehyde group,sugars containing an α-hydroxyketone group, aluminum metal and zincmetal, (ii) at least one non-reducing liquid component, and (iii) atleast one dissolved alkali metal compound selected from the groupconsisting of alkali metal hydroxides and alkali metal fluorides, at atemperature of up to about 60° C. for a time period of at least about 1second, so as to produce a wet-reduced, solid composition; (2)substantially separating the wet-reduced solid composition produced instep (1) from said liquid reduction composition; (3) drying thesubstantially separated, wet-reduced solid composition obtained in step(2); and (4) heating the dried, wet-reduced solid composition obtainedin step (3) in an oxidizing gas atmosphere at a temperature of about300°-700° C. for a time period of at least about 10 minutes.
 14. Aprocess in accordance with claim 13, wherein said inorganic supportmaterial is selected from the group consisting of alumina, titania andzirconia; said at least one reducing agent is selected from the groupconsisting of sodium borohydride, potassium borohydride, hydrazine,formaldehyde, formic acid, ascorbic acid, dextrose and aluminum metal;said at least one non-reducing liquid component is selected from thegroup consisting of water, methanol and mixtures thereof; said at leastone dissolved alkali metal is selected from the group consisting ofpotassium hydroxide, potassium fluoride, rubidium hydroxide, rubidiumfluoride, cesium hydroxide and cesium fluoride; said temperature in step(1) is about 10°-60° C.; and said time period in step (1) is about 10seconds to about 10 hours.
 15. A process in accordance with claim 14,wherein said temperature in step (1) is about 20°-50° C., said timeperiod of step (1) is about 0.02-2 hours, and the pressure in step (1)is about atmospheric.
 16. A process in accordance with claim 14, whereinsaid solid composition used in step (1) contains about 0.01-0.2 weight-%palladium, about 0.02-2 weight-% silver, and alumina as the inorganicsupport material.
 17. A process in accordance with claim 16, whereinsaid at least one reducing agent is selected from the group consistingof sodium borohydride and potassium borohydride.
 18. A process inaccordance with claim 16, wherein said at least one reducing agent ishydrazine, and said at least one dissolved alkali metal compound isselected from the group consisting of potassium hydroxide, potassiumfluoride, rubidium hydroxide and cesium hydroxide.
 19. A process inaccordance with claim 16, wherein said at least one reducing agent isformaldehyde, and said at least one dissolved alkali metal compound isselected from the group consisting of potassium hydroxide, potassiumfluoride, rubidium hydroxide and cesium hydroxide.
 20. A process inaccordance with claim 16, wherein said at least one reducing agent isselected from the group consisting of formic acid and ascorbic acid, andsaid at least one dissolved alkali metal compound is selected from thegroup consisting of potassium hydroxide, potassium fluoride, rubidiumhydroxide and cesium hydroxide.
 21. A process in accordance with claim16 wherein said at least one reducing agent is dextrose, and said atleast one dissolved alkali metal compound is selected from the groupconsisting of potassium hydroxide, potassium fluoride, rubidiumhydroxide and cesium hydroxide.
 22. A process in accordance with claim16, wherein said at least one reducing agent is aluminum metal powder,and said at least one dissolved alkali metal compound is selected fromthe group consisting of potassium hydroxide and potassium fluoride. 23.A process in accordance with claim 13, wherein the weight percentage ofsaid at least one reducing agent in said liquid reduction compositionemployed in step (1) is about 0.5-50 weight-%.
 24. A process inaccordance with claim 13, wherein said feed gas comprises acetylene asan impurity in an ethylene stream at an acetylene concentration of about1-50,000 ppm.
 25. A process in accordance with claim 13, wherein saidselective hydrogenation is carried out at a temperature of about 0° C.to about 150° C. and a pressure of about 100 to about 1,000 psig.
 26. Aprocess in accordance with claim 13, wherein step (4) is carried out inair at a temperature of about 400°-600° C. for a time period of about0.2-20 hours.